1. Field of the Invention
This invention relates to coiled tubing systems and, in certain particular aspects to arch control apparatus and methods for such systems.
2. Description of Related Art
Coiled tubing (CT) is pipe which can be run in and out of a well which may be a pipeline, tubular string, borehole, or wellbore. In certain embodiments, the CT is made of steel, titanium, plastic, or composites. The CT is stored on and spooled from a reel. In winding onto the reel, the CT is bent. Typically the CT is fed or spooled from the reel over a gooseneck or guide arch or an injector for directing the CT into a bore or well. When run into and out of a well, the CT is straightened as it comes off of the reel, bent as it goes around the guide arch, and straightened as it goes through the injector and into the well. When being pulled out of a well, the CT is bent around the guide arch, straightened as it goes towards the reel, then bent onto the reel. Thus in one trip in and out of a well, a given section of the CT is subjected to six bending and straightening events.
The CT, while it is on the reel, is like a coiled spring. Tension must be maintained on the CT as it leaves the reel at all times to control the CT on the reel and keep it from uncoiling. This tension used to control the CT on the reel is known as "reel back" tension.
Axial loads are applied to the CT both while it is being bent and straightened and while it is straight between the reel and guide arch (reel back tension) and while it is straight in a well. Repeated bending cycles can damage the coiled tubing. Internal pressure and axial loads can exacerbate this damage. The effects of this damage, known as fatigue damage, accumulate until the CT eventually fails. Failure is defined as the point at which the coiled tubing can no longer hold internal pressure, or, in extreme situations, the point at which the coiled tubing breaks. The fatigue life is the useful life of the CT before it fails due to accumulated fatigue damage.
One of the most important parameters affecting the fatigue life of the CT is the radius of bending. For a typical prior art system, e.g. as shown in FIG. 1, there are two radii of bending of coiled tubing CT. One is the radius of the reel R, and the second is the radius of the guide arch A. The reel R pulls continuously on the CT to provide the reel back tension needed to control the C on the reel. An injector I must overcome this reel back tension and control the CT as it runs in and out of the well. Of the six bending and straightening events described above, four happen at the guide arch A and two happen at the reel R. Thus, when considering ways of reducing fatigue damage, the radius of bending of the guide arch is more important than the radius of bending of the reel.
One prior art method of increasing the fatigue life (reducing the fatigue damage) involves allowing the CT to form a CT arch T as shown in FIG. 2. In this method, a small injector or reel tension device D on a reel L applies the tension to the CT needed to control the CT on the reel L. This system makes it possible the CT to form an arch between device D and an injector J. The arch T has a much larger radius of curvature than many typical guide arches. Thus this method increases the fatigue life, when compared to the system shown in FIG. 1.
A control system synchronizes the two devices, the device D with the injector J, to maintain the arch T at a constant size. If the arch T becomes too big, it may fall over. If the arch T becomes too small, it may bend or kink the coiled tubing CT or damage it in some other way. Prior art systems use depth/speed sensors to measure the CT movement at or near both the device D and the injector J. These measurements are used by a control system which synchronizes these two devices, causing them to move the same amount of CT through each device. One prior art system uses lateral load/position sensors S and N to sense the lateral load/position of the CT just above the injector J and device D. These lateral load/position measurements indicate lateral load/position of the CT as it exits the arch T. The control system(s) control the device D and the injector J to maintain the size of the arch T using these lateral load measurements; and, optionally, the depth/speed measurements at or near both the device D and the injector J are also used.
FIG. 3 shows another prior art system with the apparatus etc. of the system of FIG. 2 and with additional items. Wind forces or dynamic forces due to floating vessel movement can cause a coiled tubing arch to bend or buckle in some way. A control system error may allow the arch to become too large and lean or fall to one side. In one prior art system as shown in FIG. 3, two cables C are run from a support structure U on either side of a coiled tubing arch G. If something causes the arch G to lean or bend to one side or the other, it lays against the support cables C, preventing further bending.
U.S. Pat. No. 5,865,392 discloses a system for maintaining coiled tubing in layered coils on a reel. The system has a reel onto which the tubing is wound in layered coils and a layon roller made of compliant material maintained in physical contact with the tubing on the reel to prevent premature unwinding of the tubing during operation. The roller is moved towards and away from the tubing to maintain the roller in contact with the tubing as it is payed out and reeled onto the reel. The system has a level wind mechanism with apparatus for adjusting tension in the tubing as it passes through the level wind mechanism.
There have long been needs, recognized by the present inventor, for a system for measuring CT arch height and such a system which also uses this measurement in controlling such height.